1. Field of the Invention
This invention relates broadly to fiber optic sensors for measuring linear acceleration. More particularly, this invention relates to fiber optic sensors that employ an optical fiber coil affixed to a flexural disc.
2. Description of Related Art
The flexure or strain of an optical fiber coil affixed to a flexible disc is a well-known basis for measuring acceleration resulting from momentum forces acting on the disc in a direction normal to the disc. The amount of flexure is determined interferometrically, where interferometric measurements of strain in the optical fiber coil provide high resolution, high data rates, require low power, are immune to electromagnetic interference, and can readily be adapted for remote sensing and/or rugged applications.
The mass which provides the inertia, and hence the force to cause flexure of the disc, usually consists of the disc itself and the optical fiber coil affixed thereto. This mass is typically small. As a result, the sensitivity of the strain measurements is poor although the frequency response extends to high frequency. Additional mass can be coupled to the disc in order to improve the sensitivity of the strain measurements at the expense of frequency response. For example, U.S. Pat. Nos. 6,384,919 and 5,369,485 each describe a flexural disc fiber optic sensor having a center-supported flexural disc with additional mass that is affixed to the outer edge of the disc and disposed outside the outer circumference of the disc. US Patent Application 2005/0115320 describes a flexural disc fiber optic sensor having a center-supported flexural disc with additional mass that is affixed to the outer edge of the disc and disposed above and below the outer portion of the disc. Such additional mass improves the sensitivity of the device, but makes the device more susceptible to damage from high-g shocks.
U.S. Pat. No. 6,650,418 describes an edge-supported flexural disc that employs viscoelastic shear and compression dampers. The shear damper is in contact with the inner edge portion of the disc. The compression dampers are compressed against the fiber optic coils affixed above and below the disc. The compression dampers act to dampen extreme motions of the disc that would be otherwise caused in the disc, for example, as a result of high-g shocks. In this manner, the compression dampers protect the device against damage from such extreme motion. Disadvantageously, the shear and compression dampers of the '418 patent are difficult to manufacture at small tolerances, and thus are impractical in applications requiring significant proof mass for high sensitivity measurements.
Thus, there remains a need in the art for flexural disc fiber optic sensors that are suitable for applications requiring high sensitivity measurements while affording protection from high-g shocks that may be experienced by the sensors.